Method and apparatus for remote measurement of vibration and properties of objects

ABSTRACT

A method and apparatus is provided which employs phase or amplitude modulated electromagnetic probing waves (in optical or microwave frequency ranges or both) emitted toward a vibrating object. The optical and/or microwave probing signals remotely irradiate an object of interest. The object reflects and/or scatters the probing wave toward to a receiver. The reflected/scattered modulated signal is received with a remote sensor (receiver). Vibration causes additional phase modulation to the probing wave. At the receiving end, the signal is demodulated to extract and analyze the vibration waveform. The present invention can be utilized for nondestructive testing, monitoring of technological processes, structural integrity, noise and vibration control, mine detection, etc.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention generally relates to a method and apparatus fornondestructive testing, monitoring of technological processes,determining structural integrity, noise and vibration control, and minedetection. More specifically, the present invention relates to aphase-amplitude modulated electromagnetic wave (PAM-EW) vibrometer.

2. Related Art

Existing remote vibrometers are generally based on coherent lasergenerated signals. These devices, known as laser-doppler vibrometers,require precision and expensive optical elements (acousto-opticmodulators, gas lasers, mirrors, beam splitters, etc.) A very precise,very coherent source is required, i.e. very stable phasecharacteristics. Fine adjustments are necessary to achieve a desirableeffect. As a result, the laser-doppler vibrometers are quite expensiveand delicate instruments are best suited for laboratory use.

Another serious drawback of the conventional remote sensing devices istheir high sensitivity to unwanted vibration of thetransmitting/receiving assembly (TRA). In fact, vibrometers measure onlyrelative velocity/displacement between the vibrating object and the TRA.Since the sensitivity of the conventional laser-doppler vibrometers isvery high it is very difficult to isolate the TRA from such smallvibrations especially under field conditions. In addition to this,conventional vibrometers are susceptible to so-called cosine error. Thatis, if the incident electromagnetic wave is not precisely perpendicularto the irradiated surface, an error proportional to the cosine of theangle between the line of radiation and a normal to the surface isintroduced.

Efforts of others in this area include U.S. Pat. No. 5,883,715, toSteinlechner et al., entitled Laser Vibrometer for VibrationMeasurements; U.S. Pat. No. 5,897,494, to Flock, et al., entitledVibrometer; U.S. Pat. No. 5,495,767, to Wang, et al., entitled LaserVibrometer; and U.S. Pat. No. 4,768,381, to Sugimoto, entitled OpticalVibrometer.

None of these efforts of others teaches or suggests all of the elementsof the present invention, nor do they disclose all of the advantages ofthe present invention.

OBJECTS AND SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide aphase-amplitude modulated electromagnetic wave (PAM-EW) vibrometer.

It is an additional object of the present invention to provide a methodand apparatus for measuring vibration of a vibrating object which uses amodulated electromagnetic probing wave, wherein the vibration of thevibrating object additionally modulates the modulated probing wave.

It is another object of the present invention to provide a vibrometerwhich uses an optical source which is not necessarily coherent, forexample, an LED source.

It is even an additional object of the present invention to provide anadditional set of acoustic transmitters/receivers attached directly tothe electromagnetic wave transducer assembly to enhance performance.

These and other objects of the present invention are achieved by amethod and apparatus which employs phase or amplitude modulatedelectromagnetic probing waves (in optical or microwave frequency rangesor both) emitted toward a vibrating object. The optical and/or microwaveprobing signals remotely irradiate an object of interest. The objectreflects and/or scatters the probing wave toward to a receiver. Thereflected/scattered modulated signal is received with a remote sensor(receiver). Vibration causes additional phase modulation to the probingwave. At the receiving end, the signal is demodulated to extract andanalyze vibration waveform. The invention also employs an innovativemethod and algorithm for enhanced performance of the vibrometer by usingan additional set of acoustic transmitters/receivers attached directlyto the electromagnetic wave transducer assembly. This additional set andcorresponding data processing algorithm allow for compensation of theunwanted background (or coupled) vibration of the vibrometer and forcalibrated measurements of the displacement of the vibrating objectirradiated under an arbitrary angle. The method and apparatus of thepresent invention can be utilized for nondestructive testing, monitoringof technological processes, structural integrity, noise and vibrationcontrol, mine detection, etc.

The present invention can be used in connection with existing methodsand apparatuses for detecting land mines and detecting defects instructures. Such existing methods and apparatuses include U.S. Pat. No.5,974,881, dated Nov. 2, 1999 to Donskoy, et al. and pending U.S.application Ser. No. 09/239,133, filed Jan. 28, 1999 by Donskoy, et al.,the entire disclosures of which are expressly incorporated herein byreference.

BRIEF DESCRIPTION OF THE DRAWINGS

Other important objects and features of the present invention will beapparent from the following Detailed Description of the Invention takenin connection with the accompanying drawings in which:

FIG. 1 is a schematic view of the method and apparatus of the presentinvention.

FIG. 2 is a schematic view of the method and apparatus for compensatingfor errors arising from unwanted vibration of the transmitting/receivingassembly (TRA).

FIG. 3 a is a schematic view of an experimental set-up of the method andapparatus of the present invention.

FIG. 3 b is a graph of the results of the experiment shown in FIG. 3 a.

FIG. 4 is a schematic of a microwave vibrometer embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method and apparatus which employsphase or amplitude modulated electromagnetic probing waves (in opticalor microwave frequency ranges) emitted toward a vibrating object. Thisis shown schematically in FIG. 1. The apparatus is generally indicatedat 10. A signal is generated by the signal generator 12, and thenmodulated by the modulating device 14 which receives a modulating signalfrom the modulating generator 16. Preferably, the signal is amplitudemodulated. The optical or microwave probing signals 20 are transmittedby transmitter 18 and remotely irradiate an object 8 of interest. Theobject 8 reflects and/or scatters the probing wave 20 toward to areceiver 22, where it is received. Vibration of object 8 causesadditional phase modulation to the probing wave 20, based on the factthat object 8 is vibrating, which becomes amplitude/phase modulatedsignal 24. At the receiving end, the signal 24 is demodulated bydemodulation device 26, according to signal processing system 28, toextract and analyze vibration waveform.

The present invention can be used regardless of coherency of theemitting radiation, thus eliminating need in precision and expensiveoptical elements. A laser, or even a light emitting diode (LED) can beused as the source. The intensity is modulated at a very high frequency,for example in the GHz range. This results in significant cost reductionof the vibrometer.

The use of microwave radiation brings additional capabilities for theremote sensing, allowing for measurements of internal vibrations of theobject due to penetrating capabilities of microwave radiation. Thefrequency of the microwave radiation can be the same as the modulatingfrequency of the optical signal, thus allowing for a shared use ofelectronic circuitry for both received microwave and optical signals.

The present invention also employs an innovative method and algorithmfor enhanced performance of the vibrometer by using an additional set ofacoustic transmitters/receivers attached directly to the electromagneticwave transducer assembly. This additional set and corresponding dataprocessing algorithm allow for compensation of the unwanted background,or coupled, vibration of the vibrometer and for calibrated measurementsof the displacement of the vibrating object irradiated under anarbitrary angle.

Referring to FIG. 2, the method and algorithm for compensating forcosine and transmitter/receiving assembly (TRA) 30 vibration errors, isshown. A 3D accelerometer 32 (or any motion sensor) and a CW (continuouswave) source 34 of vibration at frequency f₀, are attached to the TRA30. The 3D sensor 32 measures three components of the TRA vibrationdisplacements: x(t), y(t), and z(t). The output of the TRA 30 isproportional to the variation in the length, L(t), between the TRA 30and the surface of the tested object 8. L(t) can be defined using FIG. 2geometry. For simplicity only the XZ-plate dependent (2D case) isconsidered:L(t)=ξ(t)/cos Θ_(xz) +x(t)sin Θ_(xz)/cos Θ_(xz) +z(t)   (1)where ξ(t) is the normal displacement of the vibrating object, andΘ_(xz) is the angle between the normal to the surface of the object 8and z-axes of the TRA 30. Here x(t) and z(t) are unwanted components ofthe output signal. The signal z(t) can be easily compensated(subtracted) since it is directly measured with the 3D sensor 32.However to compensate for x(t), the angle Θ_(xz) must be determined.This can be done using a CW vibration source 34, which causes the TRA 30to vibrate at a fixed frequency f₀ with amplitude A_(ox). Taking thisvibration into account, Eq. (1) can be re-written as:L(t)−z(t)=[ξ(t)/sin Θ_(xz) +x(t)+A _(ox) cos(2πf ₀ t)]tan Θ_(x2).  (2)

By choosing the applied vibration large enough that A_(ox)>>[ξ(t)/sinΘ_(xz)+x(t)], the output signal at the known frequency f₀ can be used toevaluate unknown angle Θ_(xz):L(t)−z(t)|_(f=f0) ≅A _(0x) tan Θ_(xz).   (3)

Thus, formula (3) can be used to evaluate the angle Θ_(xz) and knowingx(t) and z(t), which are measured with the 3D sensor 32, the truedisplacement ξ(t) can be determined using formula (1).

This algorithm can be easily extended for the 3D case, in which avibrating source generates x and y components of vibration and the 3Dsensor also measures the y component of the TRA vibration.

The apparatus of the present invention comprises an optical or microwavetransmitter, corresponding receiver, and electronics including powersupplies, signal generators, amplifiers, modulators, demodulators,acquisition and processing units.

FIG. 3 a is a schematic view of an experimental setup of the presentinvention. A laser diode 40 is used as the source of light. One suitablelaser diode is the Sharp LT-023, having a wavelength of 790 nm and 2 mWof power. Any other suitable light source can be used. Coherency of thelight source is not too important, and accordingly, even and LED couldbe used. The laser diode 40 is powered by current source 42 whichsupplies current to drive the laser 40. The current goes through a biastee 44 which is an electronic scheme which allows for the modulation ofthe current supplied to the laser diode 40. The current is modulated bythe signal from signal generator 46, at for example 250 kHz. However,for better results in practice, the modulating signal is in the GHzrange, i.e. a few GHz or higher, because the device is more sensitive athigher frequencies. The intensity of the laser signal is therebyamplitude modulated.

The modulated signal 48 is then sent at the object 50. The signal 48 isreflected or scattered by the object 50, and the reflected signal 54 isreceived by photodetector 52. In the experimental setup shown, thevibrating object 50 comprises a shaker and an accelerometer to makeactual measurements of the vibration for comparison to experimentalresults. The reflected signal 54 received by the photodetector 52 isproportional to intensity. The amplitude modulated signal 48 isadditionally modulated in phase by the vibration of the object 50 suchthat reflected signal 54 is amplitude and phase modulated. The reflectedsignal 54 is then amplified by amplifier 56 and fed to mixer 58 whichalso receives a signal from the signal generator 46. The mixer 58 mixesthese signals, the phase modulated signal and the reference signal todemodulate the reflected signal, which is sent to the spectral analyzer60.

FIG. 3 b graphically shows the frequency response of the vibratingobject measured by the laser of the present invention and as measureddirectly by the accelerometer. As can be seen, the present inventionmeasures the vibration in accordance with measurements taken directly ofa vibrating object. As the modulating frequency is increased, theresults become more accurate.

FIG. 4 is a schematic of a microwave vibrometer embodiment of thepresent invention. An oscillator or signal generator 60 generates asignal at, for example, 2.45 GHz. The signal is split by power splitter62. Part of the signal goes to mixer 76 where it will later be used. Theother part of the signal is sent to amplifier 64 where it is amplifiedand then to circulator 66 and then to antenna 68 which sends signal 70to vibrating surface 72 where it is reflected, scattered and modulated.Modulated signal 74 is also received by the antenna 68 and sent back tothe circulator 66 which decouples the signal. This signal is then sentto amplifier 76 and then to mixer 82 which is part of a heterodynescheme including second oscillator 78 which sends a signal at anintermediate frequency, for example 2.56 GHz, through power splitter 80to mixer 82. In this way, the reference signal and the reflected signalare not mixed directly, but rather each is mixed with an intermediatefrequency, which provides advantages in terms of signal to noise ratio.The signal leaving the mixer 82 is the difference of 2.56 GHz and 2.45GHz which is the intermediate frequency (IF) of 110 MHz. This signal issent to low pass filter 84 and then to amplifier 86 and then to I&Qdemodulator 88. Mixer 76 receives signals from both oscillators 60 and78 through power splitters 62 and 80 respectively, and sends them to lowpass filter 90 and then through amplifier 92 to I&Q demodulator 88. I&Qdemodulator 88 functions essentially as a mixer which demodulates thesignal into real and imaginary parts which correspond to amplitude andphase. These signals are sent through preamplifiers 94, bandpass filters96 and amplifiers 98.

The present invention can be used as a remote sensing device used forvarious applications, including, but not limited to, nondestructivetesting, characterization and monitoring of mechanical structures andcivil structures (bridges, storage tanks, etc), air- and car-frames,pipes, pressure vessels, weldments, engines, etc.

Accordingly, the present invention provides a method and apparatus thatrelates to an electromagnetic wave vibrometer which generates anelectromagnetic signal and transmits the signal at a vibrating object. Areceiver for receiving a reflected or scattered phase modulated signalfrom the vibrating object is provided and feeds the signal to ademodulator for demodulating the received signal and a signal processorfor analyzing the vibration waveform. Additionally, a method andapparatus is provided for remotely measuring properties of an objectincluding a signal generator for generating an electromagnetic signaland transmitting a signal at an object. A means for vibrating the objectis provided. The vibrating object phase modulates the transmittedsignal. A receiver picks up the reflected and scattered phase modulatedsignal and a demodulator demodulates the received signal and a signalprocessor analyzes the vibration waveform. Similarly, the presentinvention relates to methods for remotely measuring vibration andremotely determining properties of an object.

Having thus described the invention in detail, it is to be understoodthat the foregoing description is not intended to limit the spirit andscope thereof. What is desired to be protected by Letters Patent is setforth in the appended claims.

1. An electromagnetic wave vibrometer apparatus comprising: a signalgenerator for generating an optical signal; an amplitude modulator foramplitude modulating the optical signal with a microwave frequencyelectromagnetic signal to produce an amplitude modulated optical signal;a first transmitter for transmitting the amplitude modulated opticalsignal at a vibrating object; a first receiver for receiving a reflectedamplitude modulated optical signal from the vibrating object; ademodulator for demodulating the reflected amplitude modulated opticalsignal to produce a demodulated signal; and a signal processor forextracting and analyzing a vibration waveform from the demodulatedsignal. 2-5. (canceled)
 6. The apparatus of claim 1, wherein the opticalsignal is modulated by the same frequency as the microwave frequencyelectromagnetic signal.
 7. The apparatus of claim 1 wherein the signalgenerator further comprises a laser signal source.
 8. The apparatus ofclaim 1 wherein the signal generator further comprises an LED signalsource.
 9. The apparatus of claim 1 further comprising a secondvibration receiver mounted with the first receiver for compensation forunwanted background or coupled vibration.
 10. The apparatus of claim 9further comprising a second vibration transmitter mounted with the firstreceiver for calibration of the apparatus to determine an angle ofreflection.
 11. An apparatus for remotely measuring properties of anobject comprising: a signal generator for generating an optical signal;an amplitude modulator for amplitude modulating the optical signal witha microwave frequency electromagnetic signal to produce an amplitudemodulated optical signal; a first transmitter for transmitting theamplitude modulated optical signal at an object; means for vibrating theobject to modulate the amplitude modulated optical signal transmitted atthe object; a first receiver for receiving a reflected amplitudemodulated optical signal from the object; a demodulator for demodulatingthe reflected amplitude modulated optical signal using the modulatingsignal to produce a demodulated signal; and a signal processor forextracting and analyzing a vibration waveform from the demodulatedsignal. 12-15. (canceled)
 16. The apparatus of claim 11 wherein thesignal is modulated by the same frequency as the microwave frequencyelectromagnetic signal.
 17. The apparatus of claim 11 wherein the signalgenerator further comprises a laser signal source.
 18. The apparatus ofclaim 11 wherein the signal generator further comprises an LED signalsource.
 19. The apparatus of claim 11 further comprising a secondvibration receiver mounted with the first receiver for compensation forunwanted background or coupled vibration.
 20. The apparatus of claim 19further comprising a second vibration transmitter mounted with the firstreceiver for calibration of the apparatus to determine an angle ofreflection.
 21. A method of remotely measuring vibration comprising:generating an optical signal; amplitude modulating the optical signalwith a microwave frequency electromagnetic signal to produce anamplitude modulated optical signal; transmitting the amplitude modulatedoptical signal at a vibrating object; receiving a reflected amplitudemodulated optical signal from the vibrating object; demodulating thereflected amplitude modulated signal using the microwave frequencyelectromagnetic signal; and analyzing the demodulated signal. 22-25.(canceled)
 26. The apparatus of claim 21, further comprising modulatingthe optical signal at the same frequency as the microwave frequencyelectromagnetic signal.
 27. The method of claim 21 further comprisinggenerating the optical signal using a laser or a low coherent laserdiode.
 28. The method of claim 21 further comprising generating theoptical signal using an LED.
 29. The method of claim 21 furthercomprising compensating for vibration errors by determining vibrationdisplacements of the transmitter and receiver.
 30. The method of claim30 further comprising compensating for unwanted background or coupledvibration using a second vibration receiver mounted with the firstreceiver.
 31. The method of claim 30 further comprising calibrating thevibrometer to determine an angle of reflection using a second vibrationtransmitter mounted with the first receiver.
 32. A method for remotelydetermining properties of an object comprising: amplitude modulating anoptical signal with a microwave frequency electromagnetic signal toproduce an amplitude modulated optical signal; transmitting theamplitude modulated optical signal at an object; vibrating the object;receiving reflected amplitude modulated optical signals from thevibrating object; and processing the reflected amplitude modulatedoptical signals to extract information about the properties of theobject. 33-36. (canceled)
 37. The method of claim 32 further comprisingmodulating the optical signal at the same frequency as the microwavefrequency electromagnetic signal.
 38. The method of claim 32 furthercomprising generating the optical signal using a laser or a low coherentlaser diode.
 39. The method of claim 32 further comprising generatingthe optical signal using an LED.
 40. The method of claim 32 furthercomprising splitting the amplitude modulated optical signal into firstand second signals and transmitting the second signal to a demodulatorfor comparing the second signal with the received reflected amplitudemodulated optical signal.
 41. The method of claim 32 further comprisingcompensating for vibration errors by determining vibration displacementsof the transmitter and receiver.
 42. The method of claim 41 furthercomprising compensating for unwanted background or coupled vibration byproviding a second vibration receiver mounted with the first receiver.43. The method of claim 42 further comprising calibrating the vibrometerto determine an angle of reflection by providing a second vibrationtransmitter mounted with the first receiver.
 44. The apparatus of claim1, wherein the amplitude modulated optical signal is modulated in theGHz range.
 45. The apparatus of claim 11, wherein the amplitudemodulated optical signal is modulated in the GHz range.
 46. The methodof claim 21, wherein the step of amplitude modulating the optical signalcomprises amplitude modulating the optical signal in the GHz range. 47.The apparatus of claim 1, wherein the optical signal is non-coherent.48. The apparatus of claim 11, wherein the optical signal isnon-coherent.
 49. The method of claim 21, wherein the step of generatingthe optical signal comprises comprising generating a non-coherentoptical signal.